A linear actuator is a device that is used to perform an operation along a linear path. In one configuration, rotary motion is translated into linear motion by passing a lead screw through a threaded rotor of a rotary electric motor.
A can-stack linear actuator is a specific type of motor that converts electrical energy into precise and repeatable rotational movement. Converting this rotary motion into linear motion can be accomplished in several ways. The simplest way is through an external linear motor design, in which a lead screw is rigidly fixed to the rotor, and as the rotor rotates, a linear nut external to the motor will traverse the lead screw. Another way is to use a non-captive motor design which transforms this rotary motion-using threads centrally located in the rotor. As the rotor turns and the lead screw is held from rotating using an external nut, it retracts and extends depending on the rotor direction. Similarly, the captive motor design uses a lead screw and shaft assembly attached to a pinion which prevents the lead screw from rotating using a custom sleeve and is internal to the motor. Examples of this type of motor design can be found for example in U.S. Pat. No. 6,774,517 to Kowalski et al. and in U.S. Pat. No. 6,603,229 to Toye, IV, the subject matter of each of which is herein incorporated by reference in its entirety.
The present invention describes improvements that are applicable to a captive motor design, but that are also viable for all can-stack designs. In addition, the present invention describes these improvements as applied to DC stepper motors but they also lend themselves to other types of DC motors.
Current can-stack motor designs have large tolerance stack-ups and concentricity issues. Regardless of the control on the process or the accuracy of the tooling, it is difficult to create parts with identical dimensions. When designing a can-stack motor, a trade-off occurs between cost and tolerance accuracy across multiple parts. It is therefore necessary to compensate for these tolerances in the nominal dimensions of each part to gain a factor of safety in the final assembly.
FIG. 1 depicts a cross section of a typical prior art can-stack design. A sleeve bearing 12 having a bearing inner diameter 14 and a bearing outer diameter 16 is held in place front sleeve 10. A shaft (not shown) is encased in and extends through the length of front sleeve 10 and into the interior of rotor assembly 20. The sleeve bearing 12 is accommodated within a radial spacing between the front sleeve 10 and rotor assembly 20. Pole plates 22 are arranged around the magnetic rotor assembly 20. Ultimately, the design must insure proper clearances between the outer diameter of the rotor assembly 20 and the inner diameter of the pole plates 22, in order to avoid interference between them. The resulting air gap between the inner diameter of the pole plates 22 and the outer diameter of the rotor assembly 20 also can be quite large. It is highly desirable to control this air gap by minimizing the air gap as low as possible to produce a better magnetic field and thus generate more torque.
In addition, as seen in FIG. 2, the rotor assembly 20 typically comprises a plastic insert 30 that gets molded inside magnet 32. Concentricity between the magnet outer diameter, the internal threads (not shown) and the bearing journals 34 can be controlled during the molding process. The molded insert 30 has bearing journals 34 on either end that accept the inner diameter of the bearings. In order to keep the tolerance stack-up as low as possible, a tight tolerance must be held on the outer diameter of the molded bearing journal, increasing the piece price cost and complexity of the tooling. In addition, the inner diameter of the bearing needs to be ground to minimize the tolerance stack, which also adds cost to the bearing. The motor comprises two, separate front and rear sleeves which have molded bearing pockets. The diameter of the bearing pockets needs to be held to tight tolerance in order to ensure proper alignment between the front sleeve, rear sleeve, bearings and rotor. In addition to the bearing pockets, the sleeves contain the bobbin that the motor wire gets wound onto. Again, this creates multiple parts that have high cost and complex tooling.